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Airdrops were invented much later than faucets. Airdrops are made directly into the beneficiary account, while faucet earnings remain with the faucet until you withdraw them.

The difference is airdrops follow a schedule and generally end at some point, while faucets can be used at any time. In most cases, airdrops are much more valuable than what you can earn through faucets, unless you have many active referrals in a faucet.